1. Field of the Invention
This invention relates to phosphors used in x-ray intensifying screens. More particularly it relates to terbium-activated rare-earth oxysulfide phosphors.
2. Prior Art
Calcium tungstate is the major x-ray phosphor. Terbium-activated rare-earth oxysulfide phosphors are known as being more efficient x-ray phosphors than the presently used x-ray phosphors. U.S. Pat. No. 3,725,704 states that the efficiency of certain terbium-activated rare-earth oxysulfide phosphors is from about 4 to 6 times greater than calcium tungstate.
While these phosphors have good efficiency decay times are a problem. As can be appreciated an ideal material would cease to emit light instantaneous upon the cessation of the excitation media. In the x-ray usage a sort decay time is essential because excessive decay time results in poor quality exposures upon the x-ray film.
U.S. Pat. No. 3,705,858 mentions that conventionally produced terbium-activated rare-earth oxysulfide x-ray phosphors have an excess decay time. While phosphors prepared by the techniques disclosed in U.S. Pat. No. 3,705,858 are stated to have shorter decay time than the corresponding phosphors produced by conventional methods, the process disclosed is not efficient since it requires precise control and involves forming a sulfite precursor via a precipitation step, a heating step under controlled conditions followed by an ignition step. With the multitude of steps involved the procedure is both time consuming and costly.
It is believed therefore a terbium-activated rare-earth oxysulfide phosphor, prepared in a conventional manner, containing an additive which appreciably shortens the decay is an advancement in the art. It is further believed that x-ray screens utilizing such phosphor and thus enable exposed films that have improved resolution is an advancement in the art.